1. Field of the Invention
The present invention relates to an energy absorbing structure for a side portion of a vehicle body for protecting a vehicle occupant during a side collision of a vehicle.
2. Description of the Related Art
In recent years, in vehicles, particularly in automobiles, various countermeasures against side collisions have come to be adopted to improve the performance of protection of a vehicle occupant during a side collision. As one technique of this type of countermeasure against a side collision, the provision of an energy-absorbing structure in a side door has been conventionally practiced. The Jidosha Gijutsu Jireishu (Collection of Examples of Automotive Technology) No. 95352 published by the Intellectual Property Sub-Committee of the Japan Automobile Manufacturers Association, Inc. discloses a door structure in which an air-conditioning air duct for a vehicle rear seat is disposed in that vehicle compartment-side shoulder portion of a door for a vehicle front seat which is located in the vicinity of a window, so as to absorb an impact at the time of a side collision. In addition, Japanese Patent Application Laid-Open (JP-A) No. 7-89346 discloses a structure for reinforcing a vehicle door in which a guide bar (i.e., an impact beam) in a door panel is covered with a large-diameter pipe so as to absorb impact energy during a side collision. Hereafter, consideration will be given of other examples.
In the structure shown in FIG. 9, energy absorbing members 106, each formed separately and made of urethane foam, styrene foam, or the like, are respectively disposed between an upper portion 100A of a door trim 100 and a door inner panel 104 of a side door 102 and between a lower portion (armrest) 100B of the door trim 100 and the door inner panel 104.
In accordance with the above-described arrangement, when a door outer panel 108 of the side door 102 is deformed toward the inner side of a vehicle compartment during a side collision (behavior during a primary collision), the upper part of the occupant's body moves toward the door inner panel 104 side as a reaction at that time, and hits against the upper portion 100A of the door trim 100 (behavior during a secondary collision). At this time, the energy at the time when a chest portion of the occupant undergoes the secondary collision is absorbed through the elastic deformation of the energy absorbing member 106 located on the upper side, while the energy at the time when a waist portion of the occupant undergoes the secondary collision is absorbed through the elastic deformation of the energy absorbing member 106 located on the lower side.
However, with the above-described arrangement, since the bulk compressibility of the energy absorbing member 106 is set to approximately 70% by taking into consideration the material and the energy absorption characteristic, there is a disadvantage in that the thickness D.sub.1 of the energy absorbing member 106 provided for the chest becomes large in order to obtain a deformation stroke necessary for protection of the occupant.
Meanwhile, in the structure shown in FIG. 10, energy absorbing members 110 with a substantially hat-shaped cross section, each formed integrally on the door inner panel 104 and made of a thin steel sheet or the like, are respectively disposed between the upper portion 100A of the door trim 100 and the door inner panel 104 of the side door 102 and between the lower portion 100B of the door trim 100 and the door inner panel 104.
In accordance with the above-described arrangement, the absorption of energy at the time of the secondary collision of the occupant is effected in a similar manner. In addition, since the bulk compressibility of the energy absorbing member 110 can be set to 90% or more, it is possible to secure the deformation stroke necessary for protection of the occupant without enlarging the thickness (depth) D.sub.2 of the energy absorbing member 110 provided for the chest.
However, with the above-described arrangement, as shown in FIG. 11, the load is substantially fixed irrespective of an increase in displacement even from the initial period of the secondary collision of the occupant. Here, since the characteristic indicated by the chain line in the drawing, i.e., the characteristic in which the load increases at a fixed rate with an increase in displacement, is a targeted characteristic, there is room for improvement.
Incidentally, it is conceivable to provide energy absorbing members with a circular cross section (e.g., pipes made of an aluminum alloy) instead of the energy absorbing members 110 with a hat-shaped cross section (as a disclosed example, the Jidosha Gijutsu Jireishu (Collection of Examples of Automotive Technology) No. 94602 published by the Intellectual Property Sub-Committee of the Japan Automobile Manufacturers Association, Inc. is known). In this case, however, the energy absorbing member is difficult to undergo plastic deformation (difficult to crush) in an initial period of the secondary collision, and there is a tendency that the plastic deformation subsequently proceeds at a stroke. As a result, although, as a whole, the energy absorption characteristic becomes closer to the targeted characteristic than in the case where the energy absorbing members 110 with a hat-shaped cross section are used, there is room for improvement in that the rise in the load during the initial period of the secondary collision becomes sharp.